By way of example, fuel-pump meters in a fuel dispensing station usually comprise a pump enabling said fuel to be brought from a storage tank to a delivery point, in this case the tank of a vehicle, by passing through a volume meter or measurer whose function, in quite general terms, is to produce rotary motion from the flow of fuel in such a manner that one complete revolution corresponds to a known given volume of fuel passing through the meter.
An encoding system coupled to said rotary motion in association with the volume meter forms measurement apparatus suitable for forming a signal representative of the volume flow of fuel that had been dispensed, which signal is processed by a computer designed to sum the volume signal received from the encoding system so as to be able to determine the volume of fuel dispensed by the fuel-pump meter in question, and also the amount that needs to be paid, given the price per liter of fuel.
This information is then displayed to the customer on a display incorporated in the corresponding fuel pump meter.
The encoding systems most commonly used are pulse encoders suitable for delivering an electrical signal constituted by a series of pulses, each of which corresponds to the volume increment at which the volume of fuel dispensed is measured, e.g. 1 cl.
In principle, a pulse encoder comprises a coding wheel mechanically coupled to the meter and thus driven by the meter at a speed of rotation that is substantially proportional to the flow rate of the liquid. Said coding wheel carries a series of divisions, e.g. on its periphery, which in one particular embodiment may be constituted merely by slots of angular period P corresponding to said measurement volume increment.
While the coding wheel is rotating, a single sensor, e.g. an optical sensor, disposed to be able to detect the passage of said slots then provides a pulse signal that is representative of the volume flow of liquid dispensed.
This signal is sent to the computer which then establishes the volume of liquid dispensed by multiplying the total number of pulses received by the volume measurement increment.
Nevertheless, that type of known pulse encoder suffers from several drawbacks.
Firstly, it does not enable the direction of rotation of the flow meter to be defined. It is therefore not possible to detect when the direction of rotation is reversed, e.g. due to hydraulic hammer, and as a result, the encoder continues to deliver pulses even when the rotation of the coding wheel is in reverse and no volume of liquid is flowing. Since these interference pulses are taken into account by the computer along with the others, this results in an overestimate of the volume of liquid dispensed.
Also, during small-amplitude oscillations of the meter, it can happen that the level of the signal provided by the sensor remains constant, i.e. at a high level if the sensor remains in register with a slot, or at a low level if the sensor remains in register with a land between two consecutive slots. However, a low level can also be due to a fault in the sensor itself, so it is not possible with that type of encoder to distinguish between the phenomenon of small oscillations and the sensor being out of operation. Unfortunately, it is essential to be able, at all times, to detect a breakdown of the sensor.
To remedy those drawbacks, proposals have been made to add a second sensor to an existing sensor, with the second sensor likewise being suitable for detecting the same divisions of the coding wheel as the first sensor, but being offset therefrom by one-fourth of an angular period, i.e. P/4, modulo P. It can be shown that under such circumstances, by comparing the two pulse signals delivered by the two sensors, it is possible to determine the direction of rotation of the wheel and thus to eliminate the influence of large-amplitude oscillations. Nevertheless, even with the presence of two such sensors, it is still not possible to be certain of detecting the breakdown of one of the sensors, since under such circumstances the signals supplied can be confused with those that can be obtained in certain cases of small oscillations.
In order to remove all ambiguity in the interpretation of the signals, it is possible to pierce a hole through the coding wheel and to associate that hole with a third sensor. In such a configuration, any one of the sensors being out of operation can be diagnosed reliably. However, this diagnosis takes place only after the coding wheel has revolved through one complete turn, i.e. after a delay that is considered as being excessive.